1. Field of the Invention
This invention relates to a clock generator operable to extract from a digital data signal a clock signal synchronized with the data signal, wherein transitions are suppressed on the data signal when occurring outside expected time windows, thereby decreasing the incidence of phase excursions produced by noise.
2. Prior Art
Clock recovery or synchronizing devices are known wherein a variable frequency/phase oscillator is controlled in a phase-locked loop using a means detecting phase differences between the oscillator output and a data input, to produce a recovered output clock wherein transitions are synchronized with transitions on the data signal. Inasmuch as the clock can be recovered by tracking transitions in the data, it is not necessary to transmit a clock signal separately from the data in order to synchronize operation at the receiving end with operations at the sending end. The tracking, however, is more complicated than a simple phase-locked loop because due to variations resulting from the constantly-changing nature of data, the data signal may or not have a transition occurring at any particular possible time for such a transition. The prior art has provided clock recovery devices wherein the recovered clock and the data input are applied to a timing recovery circuit such that if a data transition occurs at a particular transition time, the clock frequency can be adjusted as necessary to track the data, and if no transition happens to occur in the data, the recovered clock remains stable and unadjusted. The lack of a transition in the data is not interpreted as an indication that the recovered clock frequency should be reduced.
Clock recovery devices can be applied to non-return to zero ("NRZ") data, wherein the data may be such that no transitions occur for a long time, or so-called manchester-encoded data, wherein transitions occur frequently because the data is inverted during 50% of each data bit. Tracking NRZ data is more demanding because a longer time may elapse between transitions and the clock recovery circuit may be vulnerable to noise. Clock recovery with either manchester or NRZ data requires generating a clock whose transitions coincide with the data transitions or at least maintain a constant phase relationship therewith.
NRZ data signals normally contain a direct current (DC) component, the precise level of which varies with the content of the data due to the incidence of long strings of ones or zeros. A manchester-encoded signal with similar data, however, has no DC component. The data is combined with a clock signal such that for each data bit, the signal is inverted for half the bit length, thereby avoiding any DC component which could produce difficulties in transmission. In either case a clock signal can be recovered from the data, with care taken to pass to the clock-controlling circuits only valid transitions.
The data signal can be recovered at the receiver by suitably monitoring the positive and negative going transitions in the encoded signal. The transitions are compared with a recovered clock signal produced by synchronizing an oscillator with the transitions, to reproduce the NRZ data. When a data transition occurs the clock is corrected, or adjusted more closely to match the frequency/phase of the data.
Difficulties occur when noise occurs on the data signal. The noise is misinterpreted as a transition, causing the controlled variable oscillator to drop out of synchronism with the true data signal, and possibly garbling the data due to missed or added clock transitions. Certain techniques have been proposed to minimize the occurrence of such losses of synchronism, also known as phase excursions, by attempting to discriminate good data transitions from noise pulses. Noisy data may contain extra, missing or displaced transitions, which if accepted produce clock jitter and possibly produce relatively long-lived phase excursions sufficient to result in wholly deleted or extra cycles in the recovered clock.
It is possible to reduce the sensitivity of a clock recovery circuit to extra, missing or displaced transitions by recognizing that data transitions which occur far in time from the expected transition times, i.e., synchronously with transitions on the NRZ data, are likely to be spurious and should be kept from generating correction signals. Data transitions normally occur on the rising transition of the recovered clock. Screening for transitions has been attempted according to the prior art by a number of techniques, including defining gating windows which are periodically opened at the expected time of a next clock transition. This time window can be calculated or measured according to the art, by the passage of time following the previous clock transition, i.e., following the previous transition on the recovered clock which would have corresponded to a data transition.
Accordingly, a typical prior art technique is to use a monostable multivibrator ("one shot") to open the window for passing data transitions to the clock recovery circuit at a certain time after the last clock transition. In U.S. Pat. No. 4,370,617-Brandt, a series of one shots are employed in connection with a phase-locked loop control for the purpose of suppressing rapid phase excursions. A first one shot, triggered by the previous clock transition, times out slightly prior to the next expected clock transition, thereby triggering a next one shot and opening a window during which transitions will be accepted. The window is defined by the pulse length of the second one shot. One can use a similar technique wherein a second one shot closes a window after being triggered by an accepted clock transition, the window being defined between the time-outs of the first and second one shots. In any case, at least two one shots are necessary in order to open and close a window by this technique, namely a delay for data transitions lagging the expected transition as defined by the recovered clock (after which the window closes), and a delay until just preceding the next clock transition, for data transitions leading the clock transitions (to open the window). The one shots must each have a predetermined time constant, preferably defining a narrow window. Therefore, the device is optimally useful only for a narrow frequency range.
U.S. Pat. No. 4,363,003-Osaka et al, also defines a window during which data transitions will be accepted for producing corrections to a phase-locked loop frequency generator. The Osaka invention is intended to improve on one shot windowing techniques, instead using a counter operable to count down a recovered clock signal at a frequency some multiple higher than the data transition rate, this higher rate clock thus defining a plurality of increments or subdivisions of clock intervals, which are counted up to equal the data transition clock interval. In this manner, the rate of data transitions or clocks can be divided, for example, into sixteen increments and only data transitions occurring during the last and first increment are accepted as transitions which are passed for clock correction, if necessary. The drawback of an arrangement according to Osaka is that a plurality of digital circuits are needed for the counter and for digital comparators necessary to open and close the window at the appropriate counted intervals. At high data rates, the propagation delay through the counters may be prohibitive, especially if a large number of clock subdivisions are to be counted.
U.S. Pat. No. 4,592,076-Le-Banna discloses another alternative windowing technique. When a "window mode" is selectable, the incoming data will be passed to an output only within a certain phase interval of clock transitions. Re-initialization (correction) of the phase-locked loop is inhibited except when a transition is expected. The window is produced by generating an out-of-phase clock signal, ninety degrees out of phase with the expected transitions, for opening and closing the window. The technique apparently gates data rather than clocks and the ninety degree phase delay presumably requires a two bit counter.
U.S. Pat. No. 4,425,646-Kinoshita et al also defines a window by counting down a clock signal. The clock is produced at a much higher frequency than the data transition frequency. Phase-locked loops are frequently provided with oscillators operable at a multiple of the frequency of the incoming control frequency, and operate correctly because the higher frequency is counted down to match the control frequency.
Other examples of devices for synchronizing a recovered clock to data notwithstanding the variable nature of data are disclosed in U.S. Pat. Nos. 4,644,567-Artun et al; 4,608,702-Hirzel et al; 4,320,515-Burton, Jr.; and, 4,163,946-Alberts. The disclosures of all the foregoing patents are incorporated herein.
None of the foregoing patents discloses a method and apparatus for producing a window centered on the expected clock transition time, using a minimum number of circuits in a manner which is substantially independent of the frequency of the clock. According to the present invention, an integrator and comparator are used to define such a window, thereby precluding the need for a plurality of flip flops and digital comparators in a device for counting down a multiple of the clock to define sub-interval windows. The invention also precludes the need to define a specific frequency for operation, as is characteristic of windows defined by one shots.
The integrator according to the invention can be as simple as a resistor in series with the recovered clock and a capacitor in parallel therewith, the time constant of the RC connection being much larger than the expected clock frequency such that the output is substantially a triangular wave. The comparator is preferably a high gain differential amplifier comparing the triangular wave to a reference voltage which can be fixed or variably set as close to the peaks of the triangular wave as required for the necessary noise suppression ratio. This level can be, for example, equal to the average level of the output of the phase-locked-loop recovered clock output. The invention is operable with NRZ data and with manchester-encoded data, and is simple, effective and inexpensive.